Coke, specifically, high quality coke is utilized in various industrial applications. For example, high quality coke such as anode grade coke may be used in the aluminum industry and needle grade coke may be used in the steel industry. Coking units are conventional oil refinery processing units that convert low value residual oil, from the vacuum distillation column or the atmospheric distillation column into low molecular weight hydrocarbon gases, naphtha, light and heavy gas oils, and petroleum coke. The most commonly used coking unit is a delayed coker. In a basic delayed coking process, fresh feedstock is introduced into the lower part of a fractionator. The fractionator bottoms, which include heavy recycle material and fresh feedstock, are passed to a furnace and heated to a coking temperature. The hot feed then goes to a coke drum maintained at coking conditions where the feed is cracked to form light products while heavy free radical molecules form heavier polynuclear aromatic compounds, which are referred to as “coke.” With a short residence time in the furnace, coking of the feed is thereby “delayed” until it is discharged into a coking drum. The volatile components are recovered as coker vapor and returned to the fractionator, and coke is deposited on the interior of the drum. When the coke drum is full of coke, the feed is switched to another drum and the full drum is cooled and emptied by conventional methods, such as by hydraulic means or by mechanical means.
That being said, residual oil is known to have a significant amount of asphalt and other impurities which decreases the yield of high quality coke. Thus, conventional approaches use upstream high severity hydrotreating and hydrocracking to purify the residual oil, such that the purified residual oil may be converted into high quality coke precursor, also called green coke, in the delayed coker. For example, high severity hydrotreating process may operate at hydrogen partial pressures greater than 150 bars. The green coke produced in the delayed coker may then be calcined to produce anode coke or needle coke. While the hydrotreating upstream of the delayed coker yields green coke, it is very expensive due to its high pressure requirement.